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When a drive fails and a RAID goes into reconstruction (if you are set up that way), that's when you are significantly more likely to have another drive fail due to all the extra activity across the RAID.

Don't use RAID 5. Battle Against Any Raid 5

I thought about modding parent up for mentioning the downside to RAID5 with modern hard drives but it was an AC with a score of 0 so most still wouldn't have read it.

So I'll just do it myself and link to http://www.baarf.com/

I still don't understand why so many people use RAID 5 when disks and controllers are cheap enought to do RAID 10 or multiple RAID 1 arrays instead. And if you are using enough data that RAID 10 doesn't do it for you then RAID 6 might be a OK solution but I wouldn't recommend any RAID solution unconditionally.

I will unconditionally recommend against RAID 5 though. And in a similar train of though I'll conditionally recommend against RAID 0 (though it does have valid uses it's often used by those that don't understand the down sides).

Also, as drives get bigger, they also get faster

True, but they get bigger faster. That's why there's a movement to migrate away from RAID 3-5: the risk of two drives failing within two days is too great.

I think it is in place to post the following information about files systems and the risk of data corruption:

(the information within this post is derived from a forum discussion with a user named "Kebabbert" so credits should go to him(/her never met him irl) for the excellent information on this post)

Regarding shortcomings in hardware RAID, here is a whole PhD dissertation showing that normal file systems are unreliable:

http://www.zdnet.com/blog/storage/ [...] t-risk/169

Dr. Prabhakaran found that ALL the file systems shared

...ad hoc failure handling and a great deal of illogical inconsistency in failure policy...such inconsistency leads to substantially different detection and recovery strategies under similar fault scenarios, resulting in unpredictable and often undesirable fault-handling strategies.

We observe little tolerance to transient failures;...none of the file systems can recover from partial disk failures, due to a lack of in-disk redundancy.

Regarding shortcomings in hardware RAID:

http://www.cs.wisc.edu/adsl/Public [...] fast08.pdf

"Detecting and recovering from data corruption requires protection techniques beyond those provided by the disk drive. In fact, basic protection schemes such as RAID [13] may also be unable to detect these problems.
..
As we discuss later, checksums do not protect against all forms of corruption"

http://www.cs.wisc.edu/adsl/Public [...] icde10.pdf

"Recent work has shown that even with sophisticated RAID protection strategies, the "right" combination of a single fault and certain repair activities (e.g., a parity scrub) can still lead to data loss [19]."

CERN discusses how their data was corrupted in spite of hardware RAID:

http://storagemojo.com/2007/09/19/ [...] -research/

Here is a whole site that only talks about the lacks and shortcomings in RAID-5:

http://www.baarf.com

Lacks and shortcomings in RAID-6:

http://kernel.org/pub/linux/kernel [...] /raid6.pdf

"The paper explains that the best RAID-6 can do is use probabilistic methods to distinguish between single and dual-disk corruption, eg. "there are 95% chances it is single-disk corruption so I am going to fix it assuming that, but there are 5% chances I am going to actually corrupt more data, I just can't tell". I wouldn't want to rely on a RAID controller that takes gambles :-)"

In other words, RAID-5 and RAID-6 are not safe at all and if you care about your data you should migrate to other solutions. In the past the disks were small and you were much less likely to run into problems. Today when the hard drives are big and RAID clusters are even bigger you are much more likely to run inte problems. Assume that there is a 0.00001% chance that you run into problems, if the hard drives are large and fast enough you will run into problems quite frequently.

Also if two paired drives fail in a raid1+0 it's easier to recover data out of it (sometimes just powercycle and fsck) than a parity based raid. When raid5 double deaths your data gets turned into spaghetti. Join the Battle Against Any Raid Five.

Because RAID5 is the footgun of storage solutions. It sux. It always did. It always will. Don't use it. Ever!

Now, I'll admit, with SSD a few of the arguments against it do no longer apply, but there are still enough arguments left. http://www.baarf.com/ has a nice list.

Ext3 with an ordered (default) style journal.

I believe XFS has a similar option, and Ext4 will with the next kernel, but for a home type system Ext3 should meet all of your needs, and Linux utilities still know it best.

Of course you should probably use RAID-10 too, with data disk space so cheap it is well worth it. Using the "far" disk layout, you get very fast reads, and though it penalizes writes (vs RAID 0) in theory, the benchmarks I have seen show that penalty to be smaller than the theory.

as for mkfs, large inodes probably, and when mounting use noatime.

for some anti-raid 5 propaganda:
http://www.baarf.com/

don't use raid-5? :)
cf. http://www.baarf.com/

http://www.baarf.com/

Enough is enough.
You can either join BAARF. Or not.

RAID-5 Write Penalty

"...If you later modify the data block it recalculates the parity by subtracting the
old block and adding in the new version then in two separate operations it
writes the data block followed by the new parity block. To do this it must
first read the parity block from whichever drive contains the parity for
that stripe block and reread the unmodified data for the updated block from
the original drive. This read-read-write-write is known as the RAID5 write
penalty since these two writes are sequential and synchronous the write
system call cannot return until the reread and both writes complete, for
safety, so writing to RAID5 is up to 50% slower than RAID0 for an array of
the same capacity. (Some software RAID5's avoid the re-read by keeping an
unmodified copy of the orginal block in memory.)"

RAID-5 Drive Failure
"Now if a drive in the RAID5 array dies, is removed, or is shut off data is
returned by reading the blocks from the remaining drives and calculating
the missing data using the parity, assuming the defunct drive is not the
parity block drive for that RAID block. Note that it takes 4 physical
reads to replace the missing disk block (for a 5 drive array) for four out
of every five disk blocks leading to a 64% performance degradation until
the problem is discovered and a new drive can be mapped in to begin
recovery."

Raid-5 Failure Rate Increases
As the number of disks in a RAID 5 group increases, the Mean Time Between Failures (MTBF, the reciprocal of the failure rate) can become lower than that of a single disk.

Why RAID 5 stops working in 2009

HTH,

HAND.

RAID5 with hotspare(s) is acceptable for most applications, especially with disk speeds anymore.

In RAID5, if one disk fails, and then another fails while the array is being reconstructed, you are screwed. True, disk speeds have increased, but disk capacities have increased faster than speeds, making the two-drive failure much more likely. Details at Battle Against Any Raid Five.

Two things to think about:

RAID 5 suffers catastrophic failure when two drives fail within a few hours of each other. The stress of rebuilding the replaced drive from the working ones makes this more likely to happen than some storage vendors want to admit. RAID 6 should do better because three failures in a short time are a lot less likely.

As far as I know, hardware RAID is risky if a RAID controller fails because one brand of RAID controller often can't read an array created on another brand. Software RAID on a Free operating system is more portable.

RAID-5 is EVIL! Never Ever, EVER use RAID-5, You will LOSE DATA! RAID-1 or RAID-10 only for production use.

If you are thinking about Raid-5, forget it, just stripe your drives in a RAID-0 and enjoy the performance benefits and keep frequent good backups and test your restores.

Why RAID 5 stops working in 2009
Why aren't disk reads more reliable?
End of Raid 5

finally, BAARF - Battle Against ANY Raid Five BAARF

HTH, HAND, don't cry.

Doesnt that mean that RAID 5 is the clear winner?

Too bad RAID 5 sucks.

Okay, fine, RAID 5 is good enough for data you don't care about, which would include pirated videos.

See www.baarf.com

Check out http://baarf.com/

BAARF = Battle Against Any RAID Five

Doesn't seem like a good idea anyway.

Ah, crap. I previewed and still missed it. That should be BAARF.

Add an 8 port 3ware SATA Raid controller (configured to RAID5)

I would highly recommend against a RAID-5 configuration. You're not going to save much money over RAID 1+0 and it's a ticking time bomb. See here for a cost analysis, basically you save about 33.3% by going RAID-5, but the write performance is worse than a RAID 1+0 array.

Do the math for a multi-disk failure in a RAID-5 versus a RAID 1+0 system (let's say four disks). Both the RAID-5 and RAID 1+0 system will handle a single drive failure without any problems. Now, see what the odds are of the RAID system going down when a second drive fails; in the RAID-5 case it's 100%, in the RAID 0+1 case, it's only 33.3%. More information about this is available here. I don't know about you, but having to possibly restore a terabyte array from either tape or optical media is not my idea of a good time; spend the extra cash for RAID 1+0.